FAQ: All About Sight Glasses

Also called sight window or sight port, a sight glass unit is a suitably robust window that allows convenient and/or safe visual observation of the interior of an enclosed volume.

The distinction from a general window arises from the required role. Whereas general windows are not intended to observe anything in particular, a window is referred to as a sight glass if it is intended to observe a specific and important feature within the enclosed volume. This observation target could be the fluid itself, a flow indicating device, or some phenomenon taking place within the enclosed volume.

These units are required to maintain an essentially hermetic separation between the observer and the observation target, although exceptions exist.

Read more about the different types of sight glasses.

Glass, or the medium that is transparent (or at least translucent) in the desired electromagnetic wavelength, is obviously the operational component in every sight glass unit.

Unit Body

Unit body that holds the transparent glass medium in place must also be present in all sight glasses, obviously. Most commercially available sight windows are distinct from one another in the way the unit body holds the transparent glass. Aside from simply holding the glass in place (like in any other window), the unit body is often designed to manage stresses transmitted to the glass. For pressure bearing units in particular, the unit body must distribute the sealing stresses on glass surfaces properly, while isolating the glass from the minute but still significant flexing of the pressure vessel to which the unit is mounted. In general, different pressure ratings of sight glasses arise from different unit body designs, rather than different glass materials or even thicknesses. Finally, unit body design can be an integral part of how a given unit manages corrosion protection. Unit body is not necessarily single piece.

Gaskets and Packing Material

Gaskets or packing material are always present because of the sealing requirement.

Extra features can be present as needed. For example, a sight glass used to observe a condensing process will become covered with liquid droplets, which makes observation difficult. Liquid spray ring or a window wiper is often integrated into the sight glass design in such cases. Other common inclusions are integrated fittings for specialized instrumentation, as well as lighting fixtures.

The types of metal, glass and seal material used in a sight glass are entirely dependent on the required observation mechanism, process fluid, process temperature and process pressure.

The highest cost item in an industrial unit is usually the unit body. Casting techniques are very commonly used to make the lower cost bodies. There is no real restriction on the kinds of casting methods, but investment casting methods are popular because of their relatively precise geometry control. Since the unit bodies are required to have a sealing surface present, some machining is required, as casting methods are usually not able to produce the required level of precision. Castings are particularly cost effective for very tough super alloys, as it requires the least amount of machining to finish a product while minimizing material waste.

There are several cases when unit bodies cannot be cast. The required metal might be too electropositive for casting without atmosphere control (titanium, zirconium, etc.). While such metals can certainly be cast, the required production environment for preventing oxidation during casting process, combined with the relatively rarity of such applications, makes them uneconomical to cast. Some plants also have an overall policy for only allowing forging grade structural metals in their locations. These policies are irrelevant for sight glass applications, as the weak member is the transparent window material rather than the heavy metal body, but it is easier to make the unit body out of policy compliant metals than to try to obtain exceptions to the policy. There are also non-standard sizes/designs that are not worth the investment involved in producing molds for castings. And there are sometimes delivery lead time considerations where a casting cannot be obtained quickly enough, and more expensive fabrication methods are used in order to accelerate delivery times.

When castings are not used, any subtractive machining technique can be used with virtually any metal stock. Typically, the machining starts with tube, bar or plate stock, and the stock is subjected to appropriate machining operations. For circular unit bodies, standard pipe flanges can sometimes be used as starting stock for bodies.

For the window material, commercially available float glass is used whenever possible. Float glass is cut and ground to desired shapes. If desired, they can be tempered through standard tempering processes. If very thick glass is required, rolled glass stocks can be used, as they are (or can be) available in greater thicknesses. These require not only cutting and grinding, but also lapping and polishing, adding to cost. If even greater thicknesses are required, glass sheets can be stacked and fused in order to create virtually any thickness. Virtually all glass used are of silicate composition - soda lime silicate, borosilicate, or fused quartz. Crystalline materials, like sapphire or zinc selenide, are used only for very specific needs that cannot be met by cheaper materials.

Plastic window materials are similar, except that there is usually no reason to make them thicker (nor is it easy to do so); if greater rigidity is required, silicate glass should be used instead.

Glass can be sandwiched together for greater impact resistance, at the expense of peak pressure rating for a given resulting thickness. In such arrangements, each sheet of glass layer needs to be able to hold the required pressure with requisite safety factors. The layers are bonded with a relatively soft adhesive that will shear rather than transmit the fracturing stress onto the attached layer. The trade-off for this fracture resistance is that they can only a quarter of a the pressure that can be tolerated by a single piece lens of the same total thickness.

For service in hydroxyl attack environments, glass can be coated with polymeric barriers. PTFE (Teflon) is the most popular. These coatings are not quite clear when dry, but wetting it with the process liquid makes the coating completely transparent.

Fused sight glasses are made very differently. The metal ring and glass core are placed in an atmospherically controlled furnace, and heated until the glass melts and fills out the metal ring. If the furnace cycle temperature is high enough, any form of glass can be used. After the melting out, the furnace is cooled through a controlled annealing schedule to control the stresses imposed in the glass. The pieces are finished by grinding and polishing techniques that are similar to glass polishing practices.

They are common (whether or not you referred to them as such) in everyday life. Most coffee makers or espresso machines have a type of liquid level gauge to let you see how much water you've added. Motorcycle engines have sight glasses to allow visual inspection of oil levels.

Industrial sight glasses are found in processes or plants where liquid flow inspection and control is critical. They were universal early in the industrial age where steam engines were the only mechanical motive power source and visual inspection was the only possible feedback mechanism. Industrial boiler water level inspection is still a very common application for sight glasses. There are now additional variations on that application; the sight glass units in nuclear reactor service have unit bodies made of zirconium, and those are almost certainly the most expensive sight glass bodies ever made!

Of course, there are many more applications now, reflecting the many more industries that deal with critical fluid control processes. They are used to inspect the fluid itself, as in boiler applications or petrochemical applications. Chemical and biological reactor plants often have some at critical places in order to serve as an old fashioned backup for automated instrumentation. Also, they are used with digital cameras for continuous manned monitoring of a number of critical points in a given plant.

The advent of low cost, high speed digital photography and more advanced optical measurement techniques have created new applications for specialty sight glasses. With such new tools, it became possible to literally see rapid fluid mechanical or chemical phenomena in slow motion. For such applications, researchers specify sight glass units that could operate in the pressure and temperature regimes that would make such experiments meaningful.

Laser measurements of chemical reactions are routinely performed, with the laser being separated from the chemical reaction chamber by a zinc selenide window. High pressure combustion research, fuel atomization characterization, and such, lead to challenging design requirements for sight glasses, and the limit of the conditions at which such experiments can be performed is set by sight glasses on the research reactors. There had even been interest from a major jet engine manufacturer for a sight glass unit that would permit observation into a jet engine combustion chamber! (That was a bad idea; whatever knowledge might have been gained by staring into a jet engine combustion chamber -through a digital camera, one can only hope -could not possibility be worth the cost of designing and building a unit to withstand such conditions. Did they expect to see something besides fire?)

Generating a sight glass specification is like generating any other structural engineering component specification, with one major exception. In structural engineering, it is common practice to "overengineer" an article in order to create a margin of safety. Such cases presuppose that the only risk factor is excessive mechanical load. Since sight glasses are actually interfaces into a different chemical environment, and since silicate glass strengthening often results in making them more prone to corrosion, there are other factors to consider.

In general, the first thing to do is to figure out the process upset scenarios that the sight glass will need to tolerate. Since there are multiple and often conflicting safety parameters to consider, minimizing the overall risk involves understanding the actual upset scenarios. Once that upset scenario has been specified, the following factors should be considered.

Sight glasses can be ordered online or by phone. See the buying a sight glass page for more information.